By Wednesday, more than a third of the state will have severe heatwave conditions, with a similar day forecast for Thursday.

Sydney's maximums should be moderated by coastal sea breezes most days this week, although Wednesday's top is predicted to reach 30 degrees on Wednesday and 33 degrees on Thursday.

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Sydney is set for its first heatwave of summer. Credit:Fairfax Media

But sweltering conditions will arrive for the western suburbs from Wednesday, with four days of 35 degrees or warmer predicted. Penrith and Richmond can expect 41 degrees on Thursday.

Just as a nearly stagnant weather pattern cloaked Melbourne and much of Tasmania in record-breaking heat last month, the coming week's warmth is tied to a low-pressure trough that is crawling eastwards.

The build-up of heat is triggering extreme heatwave conditions from Monday even in the Kimberley and Pilbara - two regions of Western Australian that are no strangers to hot weather.

By mid-week, a swath of heat will have spread to the coast. (See chart below.)

Western Sydney needs quite a strong sea breeze to push back the north-westerlies, and most suburbs beyond Parramatta or Bankstown won't get them until late in the weekend.

"That's why we're seeing such high temperatures over such a long time," he said.

Sydney reached its forecast top of 26 degrees on Monday just after 9.30 am, before nudging higher to 26.5 degrees just after 3 pm.

Fire risks

Along with the higher temperatures, fire risks can be expected to rise later in the week.

The current fire danger rating was "high" on Monday for Sydney, the Illawarra and the Hunter, and four other regions of the state, according to the Rural Fire Service. Similar ratings apply on Tuesday.

Mr Pippard said some regions of the state could expect severe or higher fire risks by later in the week.

At this point, however, winds are expected to be only moderate when the trough moves over Sydney on Thursday, helping to temper the threat.

That change, though, will be fairly limited.

Friday's predicted top of 28 degrees for Sydney may come with a thunderstorm, before temperatures climb back into the 30s for Saturday.

More hot weather, including tops in the mid- to high-30s for western suburbs will return by early next week.